1. Field of the Invention
The present invention relates to an image forming device and a method for forming images on recording sheets that are transported consecutively one at a time.
2. Description of the Related Art
FIG. 1 shows a conventional ink jet recording device 100 that performs printing operations. During the printing operations, recording sheets are fed from a hopper 30 one at a time and transported along a sheet path in a direction indicated by an arrow T. A head unit 2 forms an image on a recording sheet, and the recording sheet with the image formed thereon is discharged onto a tray 40.
The head unit 2 includes a plurality of recording heads 3, 4, 5, 6 for black color, magenta color, yellow color, and cyan color, respectively. The recording heads 3, 4, 5, 6 are aligned in a row in the direction T and each has a nozzle surface facing downward. Although nut shown in the drawings, each nozzle surface is formed with a plurality of nozzle lines each extending in a direction perpendicular to a sheet surface of FIG. 1. Each nozzle line includes a plurality of nozzles through which an ink droplet is ejected.
Because each recording head 3, 4, 5, 6 includes a plurality of nozzle lines, printed images have high quality and high ink dot density. However, because each recording head 3, 4, 5, 6 has more than one nozzle lines, the width of the head unit 2 can be as great as four inches in the direction T. In other words, the head unit 2 is wider than if each recording head 3, 4, 5, 6 included only a single nozzle line.
The ink jet recording device 100 can print using a monochromatic printing mode, a bicolor printing mode, or a fullcolor printing mode. In the monochromatic printing mode, monochromatic images are formed using only the recording head 3. In the bicolor printing mode, bicolor images are formed using any two of the recording heads 3, 4, 5, 6. In other words, the bicolor printing mode includes a black-magenta bicolor printing mode, a black-yellow bicolor printing mode, a black-cyan bicolor printing mode, a magenta yellow bicolor printing mode, a magenta-cyan bicolor mode, and a yellow-cyan bicolor mode. In the fullcolor printing mode, fullcolor images are formed using all of the recording heads 3, 4, 5, 6.
In order to improve printing speed, a sheet feed mechanism 31 consecutively transport a plurality of recording sheets one at a time such that each two adjacent recording sheets are separated by a distance shorter than the width W1 of the head unit 2. Also, a recording sheet is positioned at a predetermined starting position each time printing operation are completed. At the predetermined starting position, a leading edge of the recording sheet is positioned immediately upstream from the head unit 2 in the direction T, so that the next printing operations can be started quickly.
If the recording sheet was positioned further downstream than the predetermined starting position, then at least a portion of the recording sheet would be positioned beneath the head unit 2. This would lead to the following problems. For example, if next printing operations were restarted in the fullcolor mode, a portion of a fullcolor image could not be formed on the recording sheet. Also, the recording sheet beneath the head unit 2 would interfere with a maintenance mechanism (not shown), which accesses the nozzle surfaces of the recording heads 3, 4, 5, 6 from the below and performs maintenance operations on the recording heads 3, 4, 5, 6 when printing operations have not been performed for a certain time. Because of these reasons, the recording sheet needs to be upstream side of the head unit 2 when printing operations and not being performed.
However, when printing operations are repeatedly and intermittently performed in the monochromatic printing mode, pretransporting operations are performed each time printing operations are restarted. Specifically, in the monochromatic printing mode, only the recording head 3 is used. Therefore, the recording sheet that is positioned at the predetermined starting position needs to be transported by a distance L1 to a position where the leading edge of the recording sheet is positioned immediately upstream from the recording head 3. Then, actual printing is started using the recording head 3. Because actual printing is not performed until the pretransporting operations are completed, the pretransporting operations waste time and reduce printing speed.
It is an objective of the present invention to overcome the above problems, and also to provide an image forming device and a method for performing printing operations at an improved recording speed.
In order to achieve the above and other objectives, there is provided a recording device including a head unit and a buffer mechanism. The head unit includes a plurality of recording heads aligned in a first direction. The head unit performs printing operations for forming an image on a recording medium using at least one of the recording heads based on print data. The buffer mechanism transports the recording medium selectively in the first direction and a second direction opposite from the first direction. The buffer mechanism is provided upstream from the head unit in the first direction.
There is also provided a printing method including the steps of a) performing printing operations based on print data for forming an image on a recording medium, and b) positioning a subsequent recording medium at one of a plurality of starting positions corresponding to the print data.